1. Field of the Invention
The field of art to which the claimed invention pertains is the removal of sulfur oxide from a gas, particularly an FCC flue gas.
2. Background Information
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are subsequently withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking (FCC) process for the conversion of relatively high boiling hydrocarbons to lighter hydrocarbons boiling in the heating oil or gasoline (or lighter) range. The hydrocarbon feed is contacted in one or more reaction zones with the particulate cracking catalyst maintained in a fluidized state under conditions suitable for the conversion of hydrocarbons.
Due to the ever increasing concern about air pollution, great efforts have been expended in recent years toward the development of processes to reduce the pollutants introduced into the atmosphere from various industrial operations. One of the most onerous of these pollutants is sulfur dioxide which is present in the stacks of flue gases from various operations. In one such operation, the fluidized catalytic cracking (FCC) process, sulfur compounds contained in the hydrocarbon feedstock result in sulfur-containing material to be deposited on the FCC catalyst along with the carbonaceous material and thereby cause the generation of sulfur dioxide in the FCC regeneration section when the sulfur is burned off the catalyst along with the carbon deposits. This sulfur dioxide becomes a part of the regenerator flue gas and thus a pollutant when the flue gas eventually finds its way into the atmosphere.
There are many methods known to the art for removal of sulfur dioxide from stack or flue gases. There is, for example, the wet scrubbing process in which the sulfur dioxide reacts with an appropriate reactant contained in an aqueous solution or slurry sprayed into the flue gas, the sulfur thereby being removed from the system as a compound contained in the liquid phase. In another process the flue gas is passed through a fixed solid bed containing a sulfur "acceptor" with which the sulfur dioxide reacts and on which the sulfur is retained in the sulfate form, thereby being removed from the flue gas.
A prior art process for removal of sulfur dioxide from FCC flue gas highly pertinent to the present invention is that disclosed in U.S. Pat. No. 4,071,436 to Blanton, Jr., et al. In this process alumina particles are in admixture with the FCC catalyst and are circulated therewith throughout the reactor-regenerator circuit. In the regenerator the alumina reacts with sulfur dioxide to form a solid compound, which when circulated to the reactor reacts with hydrocarbons in the feedstock in the reducing environment to release the sulfur, supposedly as hydrogen sulfide. The sulfur is thereby dealt with in the FCC facilities downstream of the reactor section instead of as part of the regenerator flue gas. This reference states that it is preferred that materials such as calcium not be present in the particulate solid used for removal of the sulfur dioxide, since they simply form a noncyclical sulfur-containing solid.
U.S. Pat. No. 4,146,463 to Radford et al. discloses very broadly sulfur oxide acceptors which might be incorporated with FCC catalyst or circulated as separate particles. Among the many possibilities, this reference teaches calcium deposited on alumina as a sulfur oxide acceptor. There is no recognition in this reference, however, of the criticality of the amount and nature of such deposition. Specifically, with regard to Group IIA metals, this reference teaches very broad ranges for the amount of metal oxides which may be deposited, i.e., 25 ppm--7% as the broadest preferred range with 0.1%-0.5% as the most preferred. There is no hint in this reference to the desirability of a single complete monolayer of the deposited metal oxide on the support.
U.S. Pat. No. 4,325,811 to Sorrentino teaches the use of a separate reduction zone in a process such as that in Radford et al. in which the absorbed sulfur oxides are released from the acceptor particles. The process conditions in the reducing zone can be independently adjusted so as to optimize the removal of the sulfur oxides.
It is also known in the art that an FCC catalyst contaminated with metals such as nickel or iron from the hydrocarbon feedstock may be very effectively passivated by contacting the catalyst with a light hydrocarbon gas and hydrogen mixture at passivation reaction conditions prior to recycling the catalyst to the reactor. The passivation reaction involves the contaminating metals and serves to minimize their undesirable catalytic activity in the reaction zone. The passivation reaction is preferably carried out in a passivation reaction zone comprising a vessel in the dipleg line between the regeneration vessel and the reactor riser.
The present invention is based on the discovery of the surprising effectiveness of a sulfur oxide acceptor comprising a single complete monolayer of calcium oxide deposited on an aluminum oxide or aluminum oxide and magnesium oxide support.